A typical electric motor servo system includes an electric motor, a shaft mounted transducer for encoding the motor shaft position or motor speed, and an amplifier for providing a motor drive signal as a function of the difference between actual motor position or speed as compared to desired position or speed. In the design of a servo system it is important to avoid sources of instability in the feed-back loop. One of the more critical instability problems, which is often the limiting factor in the design, results from the torsional compliance which causes a lag between the transducer indication and the true rotor speed or position.
Normally the system designer tries to maintain the servo operation in a range below the mechanical resonance by a factor of 10 or more. For example, in a system including a two inch diameter, 0.1 inch thick, glass disk encoder and a 0.25 inch diameter by two inches long steel shaft coupling the encoder to the rotor, the expected torsional first resonance is about 1200 Hertz. Assuming a design factor of 10, the upper response frequency limit, i.e., the highest frequency of an applied signal which the servo system will reliably follow, would be about 120 Hertz. If the encoder disk is enlarged to a three inch diameter for greater accuracy, the response frequency for the system drops to below 60 Hertz. By increasing the shaft diameter to 0.5 inches to provide a stiffer coupling the response frequency limit could be increased to about 500 Hertz. From these examples it can be seen that the torsional compliance between the rotor and the encoder is often a critical factor in limiting the frequency response of a typical servo system.
An object of this invention is to provide a servo motor system wherein torsional compliance is virtually eliminated as a constraint to the frequency response in a fast response servo system.
Another object is to provide a fast response servo system which is more compact.